The present invention relates to heating, ventilating, or air conditioning (HVAC) systems, and is directed more particularly to a method for adjusting the continuous fan speed of a HVAC system that uses a standard thermostat.
HVAC systems typically include both a heating apparatus, such as a hot air furnace, and a cooling apparatus, such as an air conditioner, although a heat pump may be used to provide one or both heating and cooling. HVAC systems also include a multi-speed blower for circulating air across the heating and cooling apparatuses to the enclosure to be heated or cooled. The operation of the HVAC system as a whole is controlled by a combination furnace or fan control, which typically comprises a programmable control unit having a stored program. In the course of controlling the heating and cooling apparatuses, this control unit turns the blower on and off and generates signals which cause the blower to run at a speed that is determined by its stored program and by the settings which a user has made at his thermostat.
A typical standard thermostat includes two manually operable switches, a mode selector switch and a fan switch. The mode selector switch is a three position switch which includes a "heat" setting that causes the HVAC to operate at its heating speed, a "cool" setting which causes the HVAC to operate at its cooling speed, and an "off" setting. The fan switch is a two position switch which includes a "fan on" setting and an "auto" setting. When the fan switch is in its "fan on" position, the blower runs continuously, and provides ventilation to the enclosure. When the fan switch is in its "auto" position, the blower runs only when there is a "call for heating" or "call for cooling", and runs at one of the speeds called for by the control unit. The number of such speeds may be as few as two, one for heating and one for cooling, or as many as six, depending on the model of the HVAC system that is used.
Prior to the present invention, the speed at which the blower operated when the fan switch was in its "fan on" position was either the heating or cooling speed and could be changed at the control unit, but not at the thermostat. This is because the fan switch of a standard thermostat is connected to the control unit by only a single output wire, and could therefore only apply or not apply a current thereto. One way of allowing a user to adjust the continuous speed of the fan or blower at the thermostat is described in U.S. Pat. No. 4,146,086 (Hobbick et al), which is assigned to the same assignee as the present invention. While the thermostat described in the latter patent allows a user to adjust the continuous fan speed at the thermostat, it has the disadvantage that it requires the use of a non-standard thermostat which includes a three position fan switch and an additional wire for selecting the desired additional continuous fan speed. Since the inclusion of an additional wire is impractical in HVAC systems that have already been installed, thermostats of the latter type could only be installed in structures at the time of their construction.
In view of the foregoing, it will be seen that, prior to the present invention, there has existed a need for a method for adjusting the continuous fan speed of HVAC systems that are equipped only with standard thermostats.